PALEOZOIC STRATIGRAPHY OF ROCKY MOUNTAINS 133 



the Teton River and Crandall Creek sections marks the base of the main divi- 

 sion of the Jefferson. 



b) Member 3 was not recognized in the Antler Peak or Dead Indian Creek 

 sections, and in no known section north of Labarge Mountain does it have more 

 than a small fraction of its thickness in that locality. 



c) No part of the basal division of the Jefferson was recognized in the Logan 

 section. 



4. At the base of the Devonian system: 



a) The Silurian system, 750 or more feet thick in northern Utah, is not 

 known in Wyoming or in Montana. 



b) The Upper Bighorn dolomite, which underlies the Devonian in western 

 Wyoming and in part of southern Montana, varies greatly in thickness in that 

 region. 



c) Members 8 and 9 of the Ordovician system, which elsewhere attain a 

 net thickness of 270 feet, are not found in the Teton River section. 



d) In Montana, west of the Gallatin Range, the Devonian system rests 

 on Cambrian strata. 



To summarize points (a) to (d), the Devonian system in different parts of 

 the central Rocky Mountain region rests upon the Silurian, the Ordovician, 

 and the Cambrian, respectively. 



e) In the Teton River section an erosion surface marks the base of the 

 Darby formation. 1 



/) In the Crandall Creek section there is at this horizon a thin conglomerate 

 of small rounded pebbles of dolomite in a matrix of laminated, iron-stained 

 shale, overlain by thin lenses of very deeply iron-stained shale. The basal 

 bed varies notably in thickness within a few yards along the strike, showing 

 that it was deposited upon an irregular surface. 



It is obvious that there is a hiatus at the base of the Devonian 

 system wherever the Silurian is missing. 

 3. At the base of the Silurian system: 



a) In the Blacksmith Fork section there is an indubitable erosional dis- 

 conformity at this horizon. 



b) The brachiopod fauna above this disconformity is wholly different from 

 any found below it. 



2. Between Members 5 and 6 of the Ordovician system (between 

 the Trenton series and the Richmond series, Lower and Upper 

 Bighorn, Lower and Upper Fish Haven, Lower Bighorn and Leigh) : 



o) In the Crandall Creek, Dead Indian Creek, and Teton River sections 

 there is a well-marked erosional unconformity at this horizon. (Also at the 



1 Eliot Blackwelder, personal note. 



